Update comments docs to explain the why

```
synapse/replication/http/federation.py:72: error: Signature of "_serialize_payload" incompatible with supertype "ReplicationEndpoint"  [override]
```

changelog.d/11396.misc

@@ -0,0 +1 @@
+Refactor `backfilled` into specific behavior function arguments (`persist_events_and_notify` and downstream calls).

synapse/handlers/federation_event.py

@@ -1809,7 +1809,24 @@ class FederationEventHandler:
 
         try:
             await self.persist_events_and_notify(
-                event.room_id, [(event, context)], backfilled=backfilled
+                event.room_id,
+                [(event, context)],
+                # We should not send notifications about backfilled events.
+                inhibit_push_notifications=backfilled,
+                # We don't need to calculate the state for backfilled events and
+                # there is no need to update the forward extrems because we
+                # already know this event happened in the past if it was
+                # backfilled.
+                should_calculate_state_and_forward_extrems=not backfilled,
+                # Backfilled events get a negative stream ordering so they don't
+                # come down incremental `/sync`
+                use_negative_stream_ordering=backfilled,
+                # Backfilled events do not affect the current local state
+                inhibit_local_membership_updates=backfilled,
+                # Backfilled events have negative stream_ordering and happened
+                # in the past so we know that we don't need to update the
+                # stream_ordering tip for the room.
+                update_room_forward_stream_ordering=not backfilled,
             )
         except Exception:
             run_in_background(
@@ -1821,7 +1838,12 @@ class FederationEventHandler:
         self,
         room_id: str,
         event_and_contexts: Sequence[Tuple[EventBase, EventContext]],
-        backfilled: bool = False,
+        *,
+        inhibit_push_notifications: bool = False,
+        should_calculate_state_and_forward_extrems: bool = True,
+        use_negative_stream_ordering: bool = False,
+        inhibit_local_membership_updates: bool = False,
+        update_room_forward_stream_ordering: bool = True,
     ) -> int:
         """Persists events and tells the notifier/pushers about them, if
         necessary.
@@ -1831,8 +1853,31 @@ class FederationEventHandler:
             event_and_contexts: Sequence of events with their associated
                 context that should be persisted. All events must belong to
                 the same room.
-            backfilled: Whether these events are a result of
-                backfilling or not
+            inhibit_push_notifications: Whether to stop the notifiers/pushers
+                from knowing about the event. This should be set as True
+                for backfilled events because there is no need to send push
+                notifications for events in the past.
+            should_calculate_state_and_forward_extrems: Determines whether we
+                need to calculate the state and new forward extremities for the
+                room. This should be set to false for backfilled events because
+                we don't need to calculate the state for backfilled events and
+                there is no need to update the forward extrems because we
+                already know this event happened in the past if it was
+                backfilled.
+            use_negative_stream_ordering: Whether to start stream_ordering on
+                the negative side and decrement. This should be set as True
+                for backfilled events because backfilled events get a negative
+                stream ordering so they don't come down incremental `/sync`.
+            inhibit_local_membership_updates: Stop the local_current_membership
+                from being updated by these events. This should be set to True
+                for backfilled events because backfilled events in the past do
+                not affect the current local state.
+            update_room_forward_stream_ordering: Whether to update the
+                stream_ordering position to mark the latest event as the front
+                of the room. This should be set as False for backfilled
+                events because backfilled events have negative stream_ordering
+                and happened in the past so we know that we don't need to
+                update the stream_ordering tip for the room.
 
         Returns:
             The stream ID after which all events have been persisted.
@@ -1850,7 +1895,7 @@ class FederationEventHandler:
                     store=self._store,
                     room_id=room_id,
                     event_and_contexts=batch,
-                    backfilled=backfilled,
+                    inhibit_push_notifications=inhibit_push_notifications,
                 )
             return result["max_stream_id"]
         else:
@@ -1859,7 +1904,11 @@ class FederationEventHandler:
             # Note that this returns the events that were persisted, which may not be
             # the same as were passed in if some were deduplicated due to transaction IDs.
             events, max_stream_token = await self._storage.persistence.persist_events(
-                event_and_contexts, backfilled=backfilled
+                event_and_contexts,
+                should_calculate_state_and_forward_extrems=should_calculate_state_and_forward_extrems,
+                use_negative_stream_ordering=use_negative_stream_ordering,
+                inhibit_local_membership_updates=inhibit_local_membership_updates,
+                update_room_forward_stream_ordering=update_room_forward_stream_ordering,
             )
 
             if self._ephemeral_messages_enabled:
@@ -1867,7 +1916,7 @@ class FederationEventHandler:
                     # If there's an expiry timestamp on the event, schedule its expiry.
                     self._message_handler.maybe_schedule_expiry(event)
 
-            if not backfilled:  # Never notify for backfilled events
+            if not inhibit_push_notifications:  # Never notify for backfilled events
                 for event in events:
                     await self._notify_persisted_event(event, max_stream_token)
 

synapse/handlers/message.py

@@ -1565,12 +1565,6 @@ class EventCreationHandler:
                         errcode=Codes.INVALID_PARAM,
                     )
 
-        # Mark any `m.historical` messages as backfilled so they don't appear
-        # in `/sync` and have the proper decrementing `stream_ordering` as we import
-        backfilled = False
-        if event.internal_metadata.is_historical():
-            backfilled = True
-
         # Note that this returns the event that was persisted, which may not be
         # the same as we passed in if it was deduplicated due transaction IDs.
         (
@@ -1578,7 +1572,17 @@ class EventCreationHandler:
             event_pos,
             max_stream_token,
         ) = await self.storage.persistence.persist_event(
-            event, context=context, backfilled=backfilled
+            event,
+            context=context,
+            # Make any historical messages behave like backfilled events
+            should_calculate_state_and_forward_extrems=not event.internal_metadata.is_historical(),
+            # We use a negative `stream_ordering`` for historical messages so
+            # they don't come down an incremental `/sync` and have the proper
+            # decrementing `stream_ordering` as we import so they sort
+            # as expected between two depths.
+            use_negative_stream_ordering=event.internal_metadata.is_historical(),
+            inhibit_local_membership_updates=event.internal_metadata.is_historical(),
+            update_room_forward_stream_ordering=not event.internal_metadata.is_historical(),
         )
 
         if self._ephemeral_events_enabled:

synapse/replication/http/federation.py

@@ -47,7 +47,7 @@ class ReplicationFederationSendEventsRestServlet(ReplicationEndpoint):
                 "rejected_reason": ..,   // The event.rejected_reason field
                 "context": { .. serialized event context .. },
             }],
-            "backfilled": false
+            "inhibit_push_notifications": false
         }
 
         200 OK
@@ -69,14 +69,46 @@ class ReplicationFederationSendEventsRestServlet(ReplicationEndpoint):
         self.federation_event_handler = hs.get_federation_event_handler()
 
     @staticmethod
-    async def _serialize_payload(store, room_id, event_and_contexts, backfilled):
+    async def _serialize_payload(
+        store,
+        room_id,
+        event_and_contexts,
+        inhibit_push_notifications,
+        should_calculate_state_and_forward_extrems,
+        use_negative_stream_ordering,
+        inhibit_local_membership_updates,
+        update_room_forward_stream_ordering,
+    ):
         """
         Args:
             store
             room_id (str)
             event_and_contexts (list[tuple[FrozenEvent, EventContext]])
-            backfilled (bool): Whether or not the events are the result of
-                backfilling
+            inhibit_push_notifications: Whether to stop the notifiers/pushers
+                from knowing about the event. This should be set as True
+                for backfilled events because there is no need to send push
+                notifications for events in the past.
+            should_calculate_state_and_forward_extrems: Determines whether we
+                need to calculate the state and new forward extremities for the
+                room. This should be set to false for backfilled events because
+                we don't need to calculate the state for backfilled events and
+                there is no need to update the forward extrems because we
+                already know this event happened in the past if it was
+                backfilled.
+            use_negative_stream_ordering: Whether to start stream_ordering on
+                the negative side and decrement. This should be set as True
+                for backfilled events because backfilled events get a negative
+                stream ordering so they don't come down incremental `/sync`.
+            inhibit_local_membership_updates: Stop the local_current_membership
+                from being updated by these events. This should be set to True
+                for backfilled events because backfilled events in the past do
+                not affect the current local state.
+            update_room_forward_stream_ordering: Whether to update the
+                stream_ordering position to mark the latest event as the front
+                of the room. This should be set as False for backfilled
+                events because backfilled events have negative stream_ordering
+                and happened in the past so we know that we don't need to
+                update the stream_ordering tip for the room.
         """
         event_payloads = []
         for event, context in event_and_contexts:
@@ -96,7 +128,11 @@ class ReplicationFederationSendEventsRestServlet(ReplicationEndpoint):
 
         payload = {
             "events": event_payloads,
-            "backfilled": backfilled,
+            "inhibit_push_notifications": inhibit_push_notifications,
+            "should_calculate_state_and_forward_extrems": should_calculate_state_and_forward_extrems,
+            "use_negative_stream_ordering": use_negative_stream_ordering,
+            "inhibit_local_membership_updates": inhibit_local_membership_updates,
+            "update_room_forward_stream_ordering": update_room_forward_stream_ordering,
             "room_id": room_id,
         }
 
@@ -107,7 +143,23 @@ class ReplicationFederationSendEventsRestServlet(ReplicationEndpoint):
             content = parse_json_object_from_request(request)
 
             room_id = content["room_id"]
-            backfilled = content["backfilled"]
+            inhibit_push_notifications = content["inhibit_push_notifications"]
+            should_calculate_state_and_forward_extrems = content[
+                "should_calculate_state_and_forward_extrems"
+            ]
+            use_negative_stream_ordering = content["use_negative_stream_ordering"]
+            inhibit_local_membership_updates = content[
+                "inhibit_local_membership_updates"
+            ]
+            update_room_forward_stream_ordering = content[
+                "update_room_forward_stream_ordering"
+            ]
+
+            assert inhibit_push_notifications is not None
+            assert should_calculate_state_and_forward_extrems is not None
+            assert use_negative_stream_ordering is not None
+            assert inhibit_local_membership_updates is not None
+            assert update_room_forward_stream_ordering is not None
 
             event_payloads = content["events"]
 
@@ -132,7 +184,13 @@ class ReplicationFederationSendEventsRestServlet(ReplicationEndpoint):
         logger.info("Got %d events from federation", len(event_and_contexts))
 
         max_stream_id = await self.federation_event_handler.persist_events_and_notify(
-            room_id, event_and_contexts, backfilled
+            room_id,
+            event_and_contexts,
+            inhibit_push_notifications=inhibit_push_notifications,
+            should_calculate_state_and_forward_extrems=should_calculate_state_and_forward_extrems,
+            use_negative_stream_ordering=use_negative_stream_ordering,
+            inhibit_local_membership_updates=inhibit_local_membership_updates,
+            update_room_forward_stream_ordering=update_room_forward_stream_ordering,
         )
 
         return 200, {"max_stream_id": max_stream_id}

synapse/storage/databases/main/events.py

@@ -121,10 +121,13 @@ class PersistEventsStore:
     async def _persist_events_and_state_updates(
         self,
         events_and_contexts: List[Tuple[EventBase, EventContext]],
+        *,
         current_state_for_room: Dict[str, StateMap[str]],
         state_delta_for_room: Dict[str, DeltaState],
         new_forward_extremeties: Dict[str, List[str]],
-        backfilled: bool = False,
+        use_negative_stream_ordering: bool = False,
+        inhibit_local_membership_updates: bool = False,
+        update_room_forward_stream_ordering: bool = True,
     ) -> None:
         """Persist a set of events alongside updates to the current state and
         forward extremities tables.
@@ -137,7 +140,20 @@ class PersistEventsStore:
                 room state
             new_forward_extremities: Map from room_id to list of event IDs
                 that are the new forward extremities of the room.
-            backfilled
+            use_negative_stream_ordering: Whether to start stream_ordering on
+                the negative side and decrement. This should be set as True
+                for backfilled events because backfilled events get a negative
+                stream ordering so they don't come down incremental `/sync`.
+            inhibit_local_membership_updates: Stop the local_current_membership
+                from being updated by these events. This should be set to True
+                for backfilled events because backfilled events in the past do
+                not affect the current local state.
+            update_room_forward_stream_ordering: Whether to update the
+                stream_ordering position to mark the latest event as the front
+                of the room. This should be set as False for backfilled
+                events because backfilled events have negative stream_ordering
+                and happened in the past so we know that we don't need to
+                update the stream_ordering tip for the room.
 
         Returns:
             Resolves when the events have been persisted
@@ -159,7 +175,7 @@ class PersistEventsStore:
         #
         # Note: Multiple instances of this function cannot be in flight at
         # the same time for the same room.
-        if backfilled:
+        if use_negative_stream_ordering:
             stream_ordering_manager = self._backfill_id_gen.get_next_mult(
                 len(events_and_contexts)
             )
@@ -176,13 +192,15 @@ class PersistEventsStore:
                 "persist_events",
                 self._persist_events_txn,
                 events_and_contexts=events_and_contexts,
-                backfilled=backfilled,
+                inhibit_local_membership_updates=inhibit_local_membership_updates,
+                update_room_forward_stream_ordering=update_room_forward_stream_ordering,
                 state_delta_for_room=state_delta_for_room,
                 new_forward_extremeties=new_forward_extremeties,
             )
             persist_event_counter.inc(len(events_and_contexts))
 
-            if not backfilled:
+            # TODO: test that this actuall works
+            if stream < 0:
                 # backfilled events have negative stream orderings, so we don't
                 # want to set the event_persisted_position to that.
                 synapse.metrics.event_persisted_position.set(
@@ -316,8 +334,10 @@ class PersistEventsStore:
     def _persist_events_txn(
         self,
         txn: LoggingTransaction,
+        *,
         events_and_contexts: List[Tuple[EventBase, EventContext]],
-        backfilled: bool,
+        inhibit_local_membership_updates: bool = False,
+        update_room_forward_stream_ordering: bool = True,
         state_delta_for_room: Optional[Dict[str, DeltaState]] = None,
         new_forward_extremeties: Optional[Dict[str, List[str]]] = None,
     ):
@@ -331,11 +351,21 @@ class PersistEventsStore:
             txn
             events_and_contexts: events to persist
             backfilled: True if the events were backfilled
+            inhibit_local_membership_updates: Stop the local_current_membership
+                from being updated by these events. This should be set to True
+                for backfilled events because backfilled events in the past do
+                not affect the current local state.
+            update_room_forward_stream_ordering: Whether to update the
+                stream_ordering position to mark the latest event as the front
+                of the room. This should be set as False for backfilled
+                events because backfilled events have negative stream_ordering
+                and happened in the past so we know that we don't need to
+                update the stream_ordering tip for the room.
             delete_existing True to purge existing table rows for the events
                 from the database. This is useful when retrying due to
                 IntegrityError.
             state_delta_for_room: The current-state delta for each room.
-            new_forward_extremetie: The new forward extremities for each room.
+            new_forward_extremeties: The new forward extremities for each room.
                 For each room, a list of the event ids which are the forward
                 extremities.
 
@@ -364,7 +394,9 @@ class PersistEventsStore:
         )
 
         self._update_room_depths_txn(
-            txn, events_and_contexts=events_and_contexts, backfilled=backfilled
+            txn,
+            events_and_contexts=events_and_contexts,
+            update_room_forward_stream_ordering=update_room_forward_stream_ordering,
         )
 
         # _update_outliers_txn filters out any events which have already been
@@ -398,7 +430,7 @@ class PersistEventsStore:
             txn,
             events_and_contexts=events_and_contexts,
             all_events_and_contexts=all_events_and_contexts,
-            backfilled=backfilled,
+            inhibit_local_membership_updates=inhibit_local_membership_updates,
         )
 
         # We call this last as it assumes we've inserted the events into
@@ -1200,7 +1232,8 @@ class PersistEventsStore:
         self,
         txn,
         events_and_contexts: List[Tuple[EventBase, EventContext]],
-        backfilled: bool,
+        *,
+        update_room_forward_stream_ordering: bool = True,
     ):
         """Update min_depth for each room
 
@@ -1208,13 +1241,18 @@ class PersistEventsStore:
             txn (twisted.enterprise.adbapi.Connection): db connection
             events_and_contexts (list[(EventBase, EventContext)]): events
                 we are persisting
-            backfilled (bool): True if the events were backfilled
+            update_room_forward_stream_ordering: Whether to update the
+                stream_ordering position to mark the latest event as the front
+                of the room. This should be set as False for backfilled
+                events because backfilled events have negative stream_ordering
+                and happened in the past so we know that we don't need to
+                update the stream_ordering tip for the room.
         """
         depth_updates: Dict[str, int] = {}
         for event, context in events_and_contexts:
             # Remove the any existing cache entries for the event_ids
             txn.call_after(self.store._invalidate_get_event_cache, event.event_id)
-            if not backfilled:
+            if update_room_forward_stream_ordering:
                 txn.call_after(
                     self.store._events_stream_cache.entity_has_changed,
                     event.room_id,
@@ -1427,7 +1465,12 @@ class PersistEventsStore:
         return [ec for ec in events_and_contexts if ec[0] not in to_remove]
 
     def _update_metadata_tables_txn(
-        self, txn, events_and_contexts, all_events_and_contexts, backfilled
+        self,
+        txn,
+        *,
+        events_and_contexts,
+        all_events_and_contexts,
+        inhibit_local_membership_updates: bool = False,
     ):
         """Update all the miscellaneous tables for new events
 
@@ -1439,7 +1482,10 @@ class PersistEventsStore:
                 events that we were going to persist. This includes events
                 we've already persisted, etc, that wouldn't appear in
                 events_and_context.
-            backfilled (bool): True if the events were backfilled
+            inhibit_local_membership_updates: Stop the local_current_membership
+                from being updated by these events. This should be set to True
+                for backfilled events because backfilled events in the past do
+                not affect the current local state.
         """
 
         # Insert all the push actions into the event_push_actions table.
@@ -1513,7 +1559,7 @@ class PersistEventsStore:
                 for event, _ in events_and_contexts
                 if event.type == EventTypes.Member
             ],
-            backfilled=backfilled,
+            inhibit_local_membership_updates=inhibit_local_membership_updates,
         )
 
         # Insert event_reference_hashes table.
@@ -1638,8 +1684,20 @@ class PersistEventsStore:
             txn, table="event_reference_hashes", values=vals
         )
 
-    def _store_room_members_txn(self, txn, events, backfilled):
-        """Store a room member in the database."""
+    def _store_room_members_txn(
+        self, txn, events, *, inhibit_local_membership_updates: bool = False
+    ):
+        """
+        Store a room member in the database.
+
+        Args:
+            txn: The transaction to use.
+            events: List of events to store.
+            inhibit_local_membership_updates: Stop the local_current_membership
+                from being updated by these events. This should be set to True
+                for backfilled events because backfilled events in the past do
+                not affect the current local state.
+        """
 
         def non_null_str_or_none(val: Any) -> Optional[str]:
             return val if isinstance(val, str) and "\u0000" not in val else None
@@ -1682,7 +1740,7 @@ class PersistEventsStore:
             # band membership", like a remote invite or a rejection of a remote invite.
             if (
                 self.is_mine_id(event.state_key)
-                and not backfilled
+                and not inhibit_local_membership_updates
                 and event.internal_metadata.is_outlier()
                 and event.internal_metadata.is_out_of_band_membership()
             ):

synapse/storage/persist_events.py

@@ -35,6 +35,7 @@ from typing import (
 )
 
 import attr
+from mypy_extensions import DefaultNamedArg
 from prometheus_client import Counter, Histogram
 
 from twisted.internet import defer
@@ -112,7 +113,10 @@ times_pruned_extremities = Counter(
 @attr.s(auto_attribs=True, slots=True)
 class _EventPersistQueueItem:
     events_and_contexts: List[Tuple[EventBase, EventContext]]
-    backfilled: bool
+    should_calculate_state_and_forward_extrems: bool
+    use_negative_stream_ordering: bool
+    inhibit_local_membership_updates: bool
+    update_room_forward_stream_ordering: bool
     deferred: ObservableDeferred
 
     parent_opentracing_span_contexts: List = attr.ib(factory=list)
@@ -133,7 +137,17 @@ class _EventPeristenceQueue(Generic[_PersistResult]):
     def __init__(
         self,
         per_item_callback: Callable[
-            [List[Tuple[EventBase, EventContext]], bool],
+            [
+                List[Tuple[EventBase, EventContext]],
+                DefaultNamedArg(
+                    bool, "should_calculate_state_and_forward_extrems"  # noqa: F821
+                ),
+                DefaultNamedArg(bool, "use_negative_stream_ordering"),  # noqa: F821
+                DefaultNamedArg(bool, "inhibit_local_membership_updates"),  # noqa: F821
+                DefaultNamedArg(
+                    bool, "update_room_forward_stream_ordering"  # noqa: F821
+                ),
+            ],
             Awaitable[_PersistResult],
         ],
     ):
@@ -150,7 +164,11 @@ class _EventPeristenceQueue(Generic[_PersistResult]):
         self,
         room_id: str,
         events_and_contexts: Iterable[Tuple[EventBase, EventContext]],
-        backfilled: bool,
+        *,
+        should_calculate_state_and_forward_extrems: bool = True,
+        use_negative_stream_ordering: bool = False,
+        inhibit_local_membership_updates: bool = False,
+        update_room_forward_stream_ordering: bool = True,
     ) -> _PersistResult:
         """Add events to the queue, with the given persist_event options.
 
@@ -160,7 +178,27 @@ class _EventPeristenceQueue(Generic[_PersistResult]):
         Args:
             room_id (str):
             events_and_contexts (list[(EventBase, EventContext)]):
-            backfilled (bool):
+            should_calculate_state_and_forward_extrems: Determines whether we
+                need to calculate the state and new forward extremities for the
+                room. This should be set to false for backfilled events because
+                we don't need to calculate the state for backfilled events and
+                there is no need to update the forward extrems because we
+                already know this event happened in the past if it was
+                backfilled.
+            use_negative_stream_ordering: Whether to start stream_ordering on
+                the negative side and decrement. This should be set as True
+                for backfilled events because backfilled events get a negative
+                stream ordering so they don't come down incremental `/sync`.
+            inhibit_local_membership_updates: Stop the local_current_membership
+                from being updated by these events. This should be set to True
+                for backfilled events because backfilled events in the past do
+                not affect the current local state.
+            update_room_forward_stream_ordering: Whether to update the
+                stream_ordering position to mark the latest event as the front
+                of the room. This should be set as False for backfilled
+                events because backfilled events have negative stream_ordering
+                and happened in the past so we know that we don't need to
+                update the stream_ordering tip for the room.
 
         Returns:
             the result returned by the `_per_item_callback` passed to
@@ -170,7 +208,10 @@ class _EventPeristenceQueue(Generic[_PersistResult]):
 
         # if the last item in the queue has the same `backfilled` setting,
         # we can just add these new events to that item.
-        if queue and queue[-1].backfilled == backfilled:
+        if (
+            queue
+            and queue[-1].use_negative_stream_ordering == use_negative_stream_ordering
+        ):
             end_item = queue[-1]
         else:
             # need to make a new queue item
@@ -180,7 +221,10 @@ class _EventPeristenceQueue(Generic[_PersistResult]):
 
             end_item = _EventPersistQueueItem(
                 events_and_contexts=[],
-                backfilled=backfilled,
+                should_calculate_state_and_forward_extrems=should_calculate_state_and_forward_extrems,
+                use_negative_stream_ordering=use_negative_stream_ordering,
+                inhibit_local_membership_updates=inhibit_local_membership_updates,
+                update_room_forward_stream_ordering=update_room_forward_stream_ordering,
                 deferred=deferred,
             )
             queue.append(end_item)
@@ -241,7 +285,11 @@ class _EventPeristenceQueue(Generic[_PersistResult]):
                                 item.opentracing_span_context = scope.span.context
 
                             ret = await self._per_item_callback(
-                                item.events_and_contexts, item.backfilled
+                                item.events_and_contexts,
+                                should_calculate_state_and_forward_extrems=item.should_calculate_state_and_forward_extrems,
+                                use_negative_stream_ordering=item.use_negative_stream_ordering,
+                                inhibit_local_membership_updates=item.inhibit_local_membership_updates,
+                                update_room_forward_stream_ordering=item.update_room_forward_stream_ordering,
                             )
                     except Exception:
                         with PreserveLoggingContext():
@@ -296,15 +344,37 @@ class EventsPersistenceStorage:
     async def persist_events(
         self,
         events_and_contexts: Iterable[Tuple[EventBase, EventContext]],
-        backfilled: bool = False,
+        *,
+        should_calculate_state_and_forward_extrems: bool = True,
+        use_negative_stream_ordering: bool = False,
+        inhibit_local_membership_updates: bool = False,
+        update_room_forward_stream_ordering: bool = True,
     ) -> Tuple[List[EventBase], RoomStreamToken]:
         """
         Write events to the database
         Args:
             events_and_contexts: list of tuples of (event, context)
-            backfilled: Whether the results are retrieved from federation
-                via backfill or not. Used to determine if they're "new" events
-                which might update the current state etc.
+            should_calculate_state_and_forward_extrems: Determines whether we
+                need to calculate the state and new forward extremities for the
+                room. This should be set to false for backfilled events because
+                we don't need to calculate the state for backfilled events and
+                there is no need to update the forward extrems because we
+                already know this event happened in the past if it was
+                backfilled.
+            use_negative_stream_ordering: Whether to start stream_ordering on
+                the negative side and decrement. This should be set as True
+                for backfilled events because backfilled events get a negative
+                stream ordering so they don't come down incremental `/sync`.
+            inhibit_local_membership_updates: Stop the local_current_membership
+                from being updated by these events. This should be set to True
+                for backfilled events because backfilled events in the past do
+                not affect the current local state.
+            update_room_forward_stream_ordering: Whether to update the
+                stream_ordering position to mark the latest event as the front
+                of the room. This should be set as False for backfilled
+                events because backfilled events have negative stream_ordering
+                and happened in the past so we know that we don't need to
+                update the stream_ordering tip for the room.
 
         Returns:
             List of events persisted, the current position room stream position.
@@ -320,7 +390,12 @@ class EventsPersistenceStorage:
         async def enqueue(item):
             room_id, evs_ctxs = item
             return await self._event_persist_queue.add_to_queue(
-                room_id, evs_ctxs, backfilled=backfilled
+                room_id,
+                evs_ctxs,
+                should_calculate_state_and_forward_extrems=should_calculate_state_and_forward_extrems,
+                use_negative_stream_ordering=use_negative_stream_ordering,
+                inhibit_local_membership_updates=inhibit_local_membership_updates,
+                update_room_forward_stream_ordering=update_room_forward_stream_ordering,
             )
 
         ret_vals = await yieldable_gather_results(enqueue, partitioned.items())
@@ -350,9 +425,43 @@ class EventsPersistenceStorage:
 
     @opentracing.trace
     async def persist_event(
-        self, event: EventBase, context: EventContext, backfilled: bool = False
+        self,
+        event: EventBase,
+        context: EventContext,
+        *,
+        should_calculate_state_and_forward_extrems: bool = True,
+        use_negative_stream_ordering: bool = False,
+        inhibit_local_membership_updates: bool = False,
+        update_room_forward_stream_ordering: bool = True,
     ) -> Tuple[EventBase, PersistedEventPosition, RoomStreamToken]:
         """
+        Write a single event to the database.
+
+        Args:
+            event:
+            context:
+            should_calculate_state_and_forward_extrems: Determines whether we
+                need to calculate the state and new forward extremities for the
+                room. This should be set to false for backfilled events because
+                we don't need to calculate the state for backfilled events and
+                there is no need to update the forward extrems because we
+                already know this event happened in the past if it was
+                backfilled.
+            use_negative_stream_ordering: Whether to start stream_ordering on
+                the negative side and decrement. This should be set as True
+                for backfilled events because backfilled events get a negative
+                stream ordering so they don't come down incremental `/sync`.
+            inhibit_local_membership_updates: Stop the local_current_membership
+                from being updated by these events. This should be set to True
+                for backfilled events because backfilled events in the past do
+                not affect the current local state.
+            update_room_forward_stream_ordering: Whether to update the
+                stream_ordering position to mark the latest event as the front
+                of the room. This should be set as False for backfilled
+                events because backfilled events have negative stream_ordering
+                and happened in the past so we know that we don't need to
+                update the stream_ordering tip for the room.
+
         Returns:
             The event, stream ordering of `event`, and the stream ordering of the
             latest persisted event. The returned event may not match the given
@@ -363,7 +472,12 @@ class EventsPersistenceStorage:
         # event was deduplicated. (The dict may also include other entries if
         # the event was persisted in a batch with other events.)
         replaced_events = await self._event_persist_queue.add_to_queue(
-            event.room_id, [(event, context)], backfilled=backfilled
+            event.room_id,
+            [(event, context)],
+            should_calculate_state_and_forward_extrems=should_calculate_state_and_forward_extrems,
+            use_negative_stream_ordering=use_negative_stream_ordering,
+            inhibit_local_membership_updates=inhibit_local_membership_updates,
+            update_room_forward_stream_ordering=update_room_forward_stream_ordering,
         )
         replaced_event = replaced_events.get(event.event_id)
         if replaced_event:
@@ -379,13 +493,41 @@ class EventsPersistenceStorage:
     async def _persist_event_batch(
         self,
         events_and_contexts: List[Tuple[EventBase, EventContext]],
-        backfilled: bool = False,
+        *,
+        should_calculate_state_and_forward_extrems: bool = True,
+        use_negative_stream_ordering: bool = False,
+        inhibit_local_membership_updates: bool = False,
+        update_room_forward_stream_ordering: bool = True,
     ) -> Dict[str, str]:
         """Callback for the _event_persist_queue
 
         Calculates the change to current state and forward extremities, and
         persists the given events and with those updates.
 
+        Args:
+            events_and_contexts:
+            should_calculate_state_and_forward_extrems: Determines whether we
+                need to calculate the state and new forward extremities for the
+                room. This should be set to false for backfilled events because
+                we don't need to calculate the state for backfilled events and
+                there is no need to update the forward extrems because we
+                already know this event happened in the past if it was
+                backfilled.
+            use_negative_stream_ordering: Whether to start stream_ordering on
+                the negative side and decrement. This should be set as True
+                for backfilled events because backfilled events get a negative
+                stream ordering so they don't come down incremental `/sync`.
+            inhibit_local_membership_updates: Stop the local_current_membership
+                from being updated by these events. This should be set to True
+                for backfilled events because backfilled events in the past do
+                not affect the current local state.
+            update_room_forward_stream_ordering: Whether to update the
+                stream_ordering position to mark the latest event as the front
+                of the room. This should be set as False for backfilled
+                events because backfilled events have negative stream_ordering
+                and happened in the past so we know that we don't need to
+                update the stream_ordering tip for the room.
+
         Returns:
             A dictionary of event ID to event ID we didn't persist as we already
             had another event persisted with the same TXN ID.
@@ -448,7 +590,7 @@ class EventsPersistenceStorage:
             # device lists as stale.
             potentially_left_users: Set[str] = set()
 
-            if not backfilled:
+            if should_calculate_state_and_forward_extrems:
                 with Measure(self._clock, "_calculate_state_and_extrem"):
                     # Work out the new "current state" for each room.
                     # We do this by working out what the new extremities are and then
@@ -583,7 +725,9 @@ class EventsPersistenceStorage:
                 current_state_for_room=current_state_for_room,
                 state_delta_for_room=state_delta_for_room,
                 new_forward_extremeties=new_forward_extremeties,
-                backfilled=backfilled,
+                use_negative_stream_ordering=use_negative_stream_ordering,
+                inhibit_local_membership_updates=inhibit_local_membership_updates,
+                update_room_forward_stream_ordering=update_room_forward_stream_ordering,
             )
 
             await self._handle_potentially_left_users(potentially_left_users)

tests/replication/slave/storage/test_events.py

@@ -323,7 +323,12 @@ class SlavedEventStoreTestCase(BaseSlavedStoreTestCase):
         if backfill:
             self.get_success(
                 self.storage.persistence.persist_events(
-                    [(event, context)], backfilled=True
+                    [(event, context)],
+                    # Backfilled event
+                    should_calculate_state_and_forward_extrems=False,
+                    use_negative_stream_ordering=True,
+                    inhibit_local_membership_updates=True,
+                    update_room_forward_stream_ordering=False,
                 )
             )
         else: